1. Field of the Invention
The present invention relates generally to the art of hydraulic pressure control valves. More specifically, the present invention relates to a hydraulic modulated pressure control valve in which inlet and outlet pressures can be matched at full modulation and which includes a failsafe system to connect the valve outlet to tank in the event of a malfunction of the modulator or loss of pump pressure.
2. Description of the Prior Art
Pressure control valves of many different types are known to the art. Hydraulic pressure control valves are known which are capable of limiting outlet pressure with respect to inlet pressure and which include a means for sensing feedback pressure from the outlet side. Typically, the feedback sensing means is a valve spool area connected to the outlet pressure that is opposed by a bias spring which holds the valve spool in an open position. When the outlet feedback pressure equals the spring pressure, a pressure control level is established. Prior art devices usually provide for manual adjustment of the outlet pressure by adjustment of the biasing spring to increase or decrease the outlet pressure. These typical prior art valves have proven to be suitable for light industrial applications, but they are not satisfactory for mobile applications, such as changing, over a short time span, the controlling pressure for engaging clutches of tractors or other off-highway vehicles. Manual spring adjustment is not practical in such applications.
In addition to the above-mentioned types of pressure control valves, other pressure control valves are known to the art. For example, an electro-hydraulic driven pressure reducing valve is shown in Orme's U.S. Pat. No. 3,454,026 issued July 8, 1969 for "Pressure Feedback." The valve of this patent includes a central land on a valve spool to control the valve outlet pressure. The valve also includes a first stage servo valve to control the pressure of the incoming fluid and that going out the outlet port and two separate control chambers at each end of the servo valve spool. The valve spool is nulled by a combination of four centering springs. Second and third lands are provided at each end of the spool to isolate the various fluid pressures acting upon the valve. These second and third lands are of a reduced diameter. A passage is provided in the spool to connect the outlet to a chamber at one end of the spool to provide a feedback which in turn regulates the outlet pressure.
Another "Pressure-Regulating Valve" is shown in Spalding's U.S. Pat. No. 3,592,211 issued July 13, 1971. It employs a single control passage and a single control chamber which is driven by a first stage servo to control the fluid pressure in a second stage. A separate compensating piston is provided to keep the spool open to tank when the first stage is not driven by the second stage. The piston, of course, requires pressure to hold the spool in a biased position and leakage or contamination can cause a malfunction thereof. Differential area feedback is employed in this valve through the use of another piston acting against the spool. This combination necessitates pressure passages between the piston and the outlet port and a separate spool land between the tank and the end of the spool on which the piston acts.
Another servo-valve is described in U.S. Pat. No. 3,802,453 issued to Fleury on Apr. 9, 1974 for "Servo-Valve Responsive to Small Control Pressure." The valve includes a spool valve controlling the main pressure, one end of which is subjected to a control pressure and the other end of which is subjected to a feedback pressure. The latter is derived from the output pressure and is diminished by a resiliant counter-pressure. A low force input is placed on the spool and a high feedback pressure acts on a piston against a large spring. As the piston moves it creates an increasing load on a small spring which creates a feedback force equal to the input force. The amount of feedback force is fixed. The Fleury valve also has a land between the tank and the spool end on which the feedback spring acts.
Yet another prior art valve is shown in U.S. Pat. No. 3,856,047 issued to Takayama on Dec. 24, 1974 for "Pressure Control Valve." A spool is mounted in a housing for selectively controlling the connection between inlet and outlet ports. The spool is spring biased toward the inlet port. It includes a control chamber on the feedback side of the spool in which the inlet pressure is supplied through an orifice and then connected with another orifice, in series, but between the control chamber and the outlet port. The control pressure is higher than the outlet pressure because at acts on the full diameter of the spool end. This higher ratio feedback is a varying proportion of the outlet pressure and is proportioned to the area of the control nozzle created by the proportional solenoid. An isolating land is provided between the tank and the feedback chamber.
In addition to the aforementioned patents dealing with pressure control valves, it is known in the hydraulics art that a chamber can be connected to an inlet and that the pressure of the fluid admitted to the chamber from the inlet can be controlled or modulated. Regulation of the flow from inlet, through an orifice, to the chamber can be metered in a variety of ways. For example, a regulating means, such as a plunger, can be provided adjacent to an orifice to move in a timed relationship in response to a series of pulse signals for changing the effective flow across the orifice. In such devices, the pressure in the chamber varies with the integral of the pulses over time. A pressure regulator employing this principal is disclosed in co-pending, commonly assigned U.S. patent application, Ser. No. 06/502,699 filed June 9, 1983, now abandoned by Dale A. Knutson. The disclosure of that application is expressly incorporated herein by this reference. In the preferred embodiment of that application, the regulating means includes a solenoid activated plunger movable between first and second spaced-apart positions relative to the orifice in timed relation to each of a series of pulse signals. The frequency or width of the pulse signals may be varied to precisely control the pressure of the fluid in the chamber. To the knowledge of the present inventors, such pulse width modulated flow regulators have not been used with the type of pressure control valve to which the present invention relates.
While the aforementioned patents describe a variety of pressure control valves, they all suffer from one or more significant drawbacks. For example, pistons can malfunction due to leakage or contamination. Extra lands are required for isolation of certain pressures. A plurality of springs are required to center spools and malfunction thereof can cause valve failures. Moreover, the valves described above are relatively complex and expensive to manufacture. A pressure reducing control valve which overcomes the above-noted disadvantages of the prior art would represent a significant advance in the art. The advance would be even more significant if the resulting valve were of a simpler and safer design.